UAVs have increasing application for defence and security applications being used for many purposes including surveillance and harassment. However, UAVs may be used for a range of other purposes in civil applications including surveying and mineral exploration.
Various designs of UAV are in current use. Some are of ducted fan type in which a rotary fan, propeller or prop assembly, driven by an engine, is enclosed within a shroud. Others are of fixed wing type or helicopter type and still others are of hybrid type such as described in U.S. Pat. No. 6,270,038 assigned to Sikorsky Aircraft Corporation.
UAV engines like other engines require a cooling system to ensure satisfactory engine operation and to prevent overheating and potential failure associated therewith.
Air cooling of UAV engines is preferred, where possible, since this reduces the overall weight and failure modes of the engine, noting that liquid cooled engines, such as water cooled engines, require liquid coolant circulation systems including liquid coolant tanks or reservoirs, pumps, pipework and so on, all of which have associated mass and failure modes. Components such as pumps, and liquid coolant tanks or reservoirs—as well as liquid coolant inventory (e.g. water inventory) also add an associated cost to an overall UAV engine package. Accordingly, UAVs are optimally as lightweight as possible. Air cooling avoids the need for a liquid coolant circulation system, with its associated weight, failure modes and cost, and assists in maximizing payload of important equipment such as surveillance equipment on-board the UAV.
Whilst many UAVs take off from the ground in the manner of conventional aircraft with landing wheels, smaller UAVs do not typically take off in the manner of conventional aircraft. Rather, smaller UAVs are launched, either by a catapult or similar launch device or, possibly, by hand for lighter weight craft. However, prior to launch of a UAV, it is common for the engine, and corresponding prop assembly, to be started up ready for flight. At this stage, in which the engine may be kept running at up to full engine speed and power output for some time, heat builds up in the engine and particularly in the cylinder head of the engine. This can be problematic even at idle conditions, let alone at wide open throttle conditions.
As air cooled engines rely on convective heat transfer to provide cooling and convective heat transfer is promoted by the forced convection achieved by increasing air velocity achieved as UAV air speed increases after launch, any convective heat transfer from the engine is unforced and obviously not optimized prior to launch. Prior to launch, UAV air speed is zero and ambient airflows do not usually traverse the hotter parts of the engine surface, these parts typically being enclosed within a shroud or cowl which, though provided with ducting for cooling air, only supply this when sufficient UAV air speed is achieved. Whilst operation through rotation of the prop may cause some airflow over the engine, this will be minimal for the same reason. In such circumstances, engine overheating can become a real problem creating significant risk of engine seizure and failure.
It is an object of the present invention to provide an air cooling system for an unmanned aerial vehicle (UAV) which is operable prior to launch and at other times during operation of the UAV.